1. Technical Field
This disclosure relates generally to pumps and more particularly though not exclusively to centrifugal pumps for handling slurries.
2. Background Art
Centrifugal slurry pumps typically comprise a casing with a pumping chamber therein in which is disposed an impeller mounted for rotation on an impeller shaft. The impeller shaft enters the pumping chamber from the rear side, or drive side, of the pump housing. A discharge outlet extends tangentially from the periphery of the pump housing and provides for the discharge of fluid from the pump chamber.
One form of conventional pump casing for a centrifugal pump is illustrated in FIGS. 1 to 4. FIGS. 1 and 2 are perspective illustrations of the pump casing shown from slightly different front side angles. FIG. 3 is a sectional side elevation of the casing and FIG. 4 is a sectional view along the line X-X of FIG. 3.
The pump casing 10 includes a peripheral wall portion 12 having a pumping chamber 14 therein and opposed sides 15 and 16 (FIG. 4). During use, an impeller is mounted for rotation within the pump casing. An inlet opening to the pumping chamber 14 is provided on one side of the casing and a drive shaft to which the impeller is mounted extends through the other side. The pumping chamber 14 in the region of the peripheral wall portion 12 is of a volute shape, offset circular shape or any other suitable shape. A discharge outlet 13 extending from the peripheral wall portion 14, there being a cutwater 19 which in use generally serves to divide the discharge outlet flow from the pumping chamber recirculation flow.
In other forms of centrifugal pumps an outer housing may be provided which encases the pump casing which is shown in FIGS. 1 to 4. Throughout this specification when the term “pump casing” is used, it refers to a chamber which surrounds a pump impeller and in which the impeller can rotate in use. In unlined pumps, the “pump casing” also is the exterior casing of the pump. In a lined pump, the “pump casing” can be a lining or liner (also known as a volute), which is itself surrounded by an exterior casing structure. Unlined pumps typically find application in low wear situations, for example in use to pump liquids or non-abrasive solid-liquid mixtures. In lined pumps, the liner or volute is a wear part which is exposed to the movement of an abrasive slurry during use, and which eventually requires replacement, and the exterior casing or shell of the pump remains undamaged.
The pump casing may be formed from hard metal such as a white iron, or an elastomeric material, such as rubber. The pump casing may further include side liners mounted at respective sides 15, 16 of the pump casing 10. As is best seen in FIG. 4, in a conventional pump casing the cutwater 19 is arch shaped, having transition zones 17 in the form of tapering blend sections extending from the ends of the arch-shaped cutwater between the discharge outlet 13 and the pumping chamber 14, in the region of the peripheral wall portion 12. The cutwater 19 is that part of the casing which is the closest to the outer periphery of the impeller, the function of which is to assist the distribution of fluid flow into the discharge outlet 13 and to minimise the recirculation around the circumferential region of the pumping chamber (that is, the region between the inner surface of the peripheral wall portion 12 and the outer circumference of an impeller when located within the pumping chamber).
In use, a centrifugal slurry pump is required to operate over a wider range of flows and pressure heads during its normal operation, and may even be driven via a variable speed drive to achieve a wide operational range of flow and pressure. Depending on the pump speed, the slurry flow and particles which exit the rotating impeller into the volute region will either exit the volute into the discharge outlet (flow B in FIG. 3) or the flow and particles will recirculate around the volute (flow A in FIG. 3). The best efficiency point (BEP) of a centrifugal slurry pump is defined as the flow that produces the highest operational efficiency at one particular rotational speed. At the BEP the amount of recirculation around the volute (flow A) is minimal as the flow approaching the cutwater is at the correct flow angle relative to the cutwater, such that the cutwater divides the flow more uniformly with smooth streamlines on either side of the cutwater.
Centrifugal slurry pumps are typically not used in mining application at flows higher than the BEP flow, due to the accelerated erosive wear of the components which may occur. Instead, a centrifugal slurry pump is selected such that the flow is between 30 and 100% of the BEP flow at any one operating speed. Under these operating conditions, the degree of recirculation (flow A) around the volute can increase, which can also cause more turbulence within the volute, particularly at the cutwater region of the volute. Since the flow approaching the cutwater is more turbulent, the velocity will not be uniform, nor have a smooth flow to match the cutwater angle.
The recirculating flow in the volute is influenced by the cutwater 19 and also by the transition zones 17 shown in FIGS. 3 and 4. With an arch shaped transition region, in operation it is possible that two large swirling flow vortex patterns will be created on either side of the volute which then interact at the cutwater region, and then further downstream of the cutwater region at generally around the centreline of the volute. These vortex flows can result in the slurry particles having a higher energy and velocity, resulting in wear and erosion of the material in and around the cutwater region because this region is closest to the impeller and also is the dividing point for the flows A and B.
As mentioned earlier, centrifugal slurry pumps may, in one form typically comprise an outer casing with an internal liner moulded from a wear resisting elastomer compound. In this form, both the outer casing and the liner are traditionally manufactured in two parts or halves which are held together with bolts positioned at the external periphery of the casing. The two parts join along a plane which is generally perpendicular to the axis of rotation of the pump impeller.
When assembled, the two parts form a housing having a front side with an inlet therein and a rear side, the two parts defining a pumping chamber therein in which is disposed an impeller mounted for rotation on an impeller shaft. In some embodiments the impeller shaft enters the pumping chamber from the rear side and an outlet is provided at a peripheral side edge or wall portion of the housing.
As described earlier, the cutwater separates the flow circulating in the pumping chamber from the flow discharging through the outlet. The flow can have pressure fluctuations imposed on it as a result of the impeller pumping vanes passing the cutwater as the impeller rotates. The cutwater has unequal pressure distribution on its opposing sides due to the nature of the flow. Pressure pulses can cause the rubber to vibrate which results in fretting on the contact surfaces of the rubber liners and/or of the rubber inside the pump casing. Vibration in rubber also causes hysteresis losses within the rubber which can lead to breakdown of the rubber and a reduction in its strength due to a build-up of temperature from the losses.